1. Field of the Invention
The present invention relates to capacitors used for semiconductor devices and semiconductor devices using the capacitors.
2. Description of the Related Art
Capacitors used for semiconductor integrated circuits and the like have conventional structures in each of which a dielectric such as silicon oxide is sandwiched between electrodes made of a metal or a semiconductor such as silicon with high conductivity. However, as circuits are miniaturized, the use of materials having higher permittivity (high-k materials) than silicon oxide (relative permittivity around 4) has begun to be studied.
A problem in the use of such a high-k material is generally that the electron affinity (the energy difference between a vacuum level and a bottom of a conduction band) of such a high-k material is large. For example, according to Reference 6, the electron affinity of silicon oxide is 0.9 eV, while the electron affinity of hafnium oxide that is one of high-k materials is 2.5 eV.
For that reason, for example, the potential barrier of a junction between n-type silicon and silicon oxide is 3.5 eV, whereas the potential barrier of a junction between n-type silicon and hafnium oxide is 1.5 eV (note that the potential barrier is not necessarily equal to the difference between the work function of n-type silicon and the electron affinity of a dielectric, due to polarization in the junction).
Even with the potential barrier of about 1.5 eV, if the thickness of a dielectric is 5 nm or more, a noticeable problem does not occur in an insulating property, but when the thickness of a dielectric is desirably 3 nm or less along with miniaturization of a circuit, the amount of a tunnel current due to a quantum effect becomes large, so that sufficient function as a capacitor is not met, which is a problem.
In addition, tantalum oxide, barium strontium titanate, lead titanate, lead zirconate, barium zirconate, or the like having a higher permittivity has a potential barrier of 0.5 eV or lower in a junction with n-type silicon, and thus a sufficient insulating property is difficult to be obtained even when the thickness of a dielectric is 10 nm or more.
In general, when the potential barrier is 1 eV or more, thermal excitation can be neglected. As long as a dielectric has an appropriate thickness, the dielectric can be practically used as a capacitor (see Reference 6). On the contrary, when the potential barrier is less than 1 eV, the dielectric is not suitable for a capacitor.
Such problems are thought to be solved by the use of a material having a higher work function than n-type silicon (its work function is 4.0 eV), that is, a metal such as gold (its work function is 5.1 eV), palladium (its work function is 5.2 eV), platinum (its work function is 5.4 eV). In other words, this is because the potential barrier of a dielectric, between the dielectric and a metal having a work function higher than that of n-type silicon by 1 eV, is higher by 1 eV than that of n-type silicon.
However, such metals having a high work function are expensive, and there is no other practical method for forming a thin film of such a metal, except a physical formation method such as a sputtering method. By a physical formation method, for example, it is difficult to form a film with a good coverage, over an object having a peculiar shape such as a trench-type capacitor or a stack-type capacitor used in DRAM or the like.
Also, there is a report that such a metal having a high work function is easily reacted with a high-k material. Further, for example, because platinum promotes oxygen release from an oxide, there is a need that a barrier layer of another material is formed separately between platinum and a high-k material.